The present invention relates to X-ray imaging apparatus, and more specifically to means for suppressing high frequency electrical noise produced by an X-ray tube of the apparatus.
The X-ray imaging apparatus includes a vacuum tube, having a cathode and an anode, which emit X-rays during operation. The cathode comprises a tungsten thermionic emitting source and focusing surfaces. The cathode assembly of an X-ray tube typically includes a filament that heats the assembly to an operating temperature. Upon the application of an applied voltage potential, the thermionically emitted electrons traverse the vacuum gap between the cathode and the anode, impacting the anode and thereby generating X-rays. X-ray tubes which are typically used for medical diagnostic imaging are operated at very high anode-cathode voltages, typically 40,000 to 150,000 volts.
This range of operating voltages produces intense electric fields in the vacuum between the anode and the cathode. Such fields are intensified by sharp edges and particles on the surface of the electrodes. If the electric field intensity becomes high enough, a high voltage instability, or discharge, called a "tube spit" occurs which partially vaporizes the irregularity that produced the high field intensity. If the new surface following the vaporization is not smooth enough to lower the electric field to a sufficiently low intensity, the process repeats itself at a high frequency until the surface will support the high voltage. This process is often called "seasoning" in the X-ray tube art and occurs occasionally throughout the life of an X-ray tube providing a means by which the tube cleans itself.
Unfortunately, the high voltage discharges excite the natural resonances of the electrical circuits inside the tube casing. The resulting high frequency oscillations, typically in the range of 100 megahertz, are conducted and radiated into electronic equipment in the vicinity of the X-ray apparatus. These oscillations often have very high power and can cause permanent damage to sensitive electronic components and, more commonly, misoperation of the electronic equipment.
The traditional method for reducing the effect of tube spits on nearby electronic equipment is to prevent the electrical noise from entering the equipment by enclosing the circuitry in metal housings and by careful design of a grounding system for those housings. Although such measures for reducing the effects of the electrical noise from tube spits are helpful to a degree, they often are not effective against very intense tube spits.